1. Field of the Invention
The present invention relates to a circuit for charging a secondary battery constituted by a plurality of serially connected battery cells. More particularly, the present invention relates to a charging circuit for a secondary battery, which is capable of preventing current from leaking out of each battery cell and of accurately detecting voltage of each battery cell.
2. Description of the Related Art
Cellular phones, digital cameras, and other portable electronic devices these days are installed with chargeable secondary batteries as the power sources. Lithium ion batteries are mostly used as these secondary batteries, and especially a charging circuit in which a plurality of fuel cells composed of the lithium ion batteries are connected serially or in parallel is widely used. For such secondary batteries, the demand for increase in the charging capacities and densities has been growing.
In order to supply power to the charging circuit for charging such secondary battery, a charger is connected to the charging circuit. However, the secondary battery might enter an overcharge state due to excess voltage or excess current added from the charger to the secondary battery. For example, when a lithium ion battery enters the overcharge state, an irregular amount of voltage or current is added thereto, and as a result the battery produces heat, causing deformation thereof and, in some cases, bursting or firing.
Therefore, in order to solve the above problems, an overcharge protection circuit has been developed. When charging a plurality of serially connected lithium ion battery cells, this overcharge protection circuit monitors the total voltage of the plurality of lithium ion battery cells, and, when the total voltage exceeds a predetermined voltage, determines that the lithium ion battery cells are in the overcharge state and therefore stops charging the lithium ion battery cells.
However, in this overcharge protection circuit that determines based on the total voltage of the plurality of serially connected lithium ion battery cells whether these lithium ion battery cells are in the overcharge state, whether each battery cell is overcharged or not cannot be determined accurately if the voltage of each battery cell fluctuates. For instance, in a charging circuit that has three serially connected lithium ion batteries having a normal value of 4.2V in a fully-charged state, suppose that charging of each battery cell is stopped when the normal value of any of the battery cells becomes equal to or higher than 4.4V.
In this case, when the voltages of the three battery cells are equally 4.4V beyond the fully-charged state, the total voltage of the battery cells is 13.2V, which is determined as the overcharge state and no problem arises. However, if the voltages of the battery cells are different from one another when the batteries are deteriorated or the voltages fluctuate, the overcharge state of each battery cell cannot be detected accurately. In other words, when the voltages of the three battery cells are 4.2V, 4.3V and 4.5V, respectively, the total voltage is 13.0V. Although no abnormality is detected because this total voltage is equal to or lower than 13.2V, which is the criterion of the overcharge state, the battery of 4.5V voltage exceeds the normal value of the fully-charged state, which means that this battery is in the overcharge state.
In order to detect the overcharge state of each battery cell, it is necessary to monitor the voltage of each battery cell to determine whether it is in the overcharge state. For example, FIG. 4 shows a charging circuit which causes an excess voltage detector corresponding to each of a plurality of connected lithium ion battery cells to directly detect the voltage of the corresponding lithium ion battery cell, to determine whether the detected lithium ion battery cell is in the overcharge state.
In the charging circuit shown in FIG. 4, excess voltage detectors 4 to 6 that detect, respectively, voltages of three serially connected battery cells 1 to 3 (the serial connection of the battery cells 1 to 3 is referred to as “battery”) are connected in parallel with the battery cells 1 to 3. Moreover, there is provided a charging controller 7 that determines whether the battery cells 1 to 3 are in the overcharge states on the basis of the voltages detected by the excess voltage detectors 4 to 6, and then controls ON/OFF of a charging switch part 8.
In addition, the positive electrode of the battery is connected to an input terminal A1 via the charging switch part 8, and the negative electrode to an input terminal A2. A charger or the like is connected to the input terminals A1, A2, and thereby these input terminals A1, A2 are used for charging the battery cells 1 to 3 constituting the battery, that is, for supplying electric charges to the battery cells 1 to 3. As shown in FIG. 4, the excess voltage detectors 4 to 6 that are connected in parallel with the battery cells 1 to 3 and detect the voltages of the battery cells 1 to 3, respectively, have a structure in which two resistances are connected in series, and references of shunt regulators are connected to the connecting points of these resistances. These excess voltage detectors 4 to 6 determine whether the detected battery voltages exceed a previously set reference voltage.
An example of a process for determining the overcharge state in the charging circuit of FIG. 4 is described. First of all, when charging voltage to the battery, the charging switch part 8 is turned ON and electric charges are supplied to the battery cells 1 to 3 through the input terminals A1, A2. The excess voltage detectors 4 to 6 determine whether any of the detected voltages of the battery cells 1 to 3 exceeds the reference voltage indicating the overcharge state and, when it exceeds the reference voltage, transmits an excess voltage signal to the charging controller 7.
When the charging controller 7 receives the excess voltage signal from any of the excess voltage detectors 4 to 6, the charging controller 7 performs control to turn the charging switch part 8 OFF and then stops charging the voltage to the battery by terminating the supply of the electric charges to the battery cells 1 to 3. When the voltages of the battery cells 1 to 3 that are detected by the excess voltage detectors 4 to 6 do not exceed the reference voltage indicating the overcharge state the charging switch part 8 remains turned ON.
Not only the circuit of FIG. 4 that has the abovementioned configuration, but also a charging circuit described in Japanese Patent Application Publication No. 2007-14091 also has been developed as the conventional technology for monitoring a voltage of each battery cell and directly detecting the voltage of the battery cell by means of a voltage comparator to determine the overcharge state.
Incidentally, because the voltage of each battery is directly detected in the charging circuit of FIG. 4 or Japanese Patent Application Publication No. 2007-14091 that determines the overcharge state by monitoring each battery cell, when the voltages of the battery cells vary from one another as a result of deterioration or the like of any of the batteries, the consumed current might leak out of a circuit part to each battery cell, the circuit part determining whether each battery cell is in the overcharge state or not. As a result, the consumed current from the circuit part charges and discharges the battery cells, and consequently the circuit part itself damages the voltage balance of the batteries.
Suppose, for instance, there is the charging circuit in which the excess voltage detectors 4 to 6 for detecting the voltages of the three serially connected lithium ion battery cells 1 to 3 are directly connected in relation to the battery cells 1 to 3, as shown in FIG. 4. In this case, when the following [Expression 1] is satisfied based on a relationship among a current I1a flowing in the excess voltage detector 4 corresponding to the battery cell 1, currents I2a and I2b flowing in the excess voltage detector 5 corresponding to the battery cell 2, and currents I3a and I3b flowing in the excess voltage detector 6 corresponding battery cell 3, charge/discharge currents I1 and I2 do not flow to each of the battery cells.I2a+I2b=I1a I3a+I3b=I2a  [Expression 1]
However, when the voltages of the battery cells 1 to 3 are not equal due to deterioration of any of the batteries or fluctuation of the voltages of the battery cells, or when the abovementioned [Expression 1] is not satisfied by a current value sent from the charging controller 7, the charge/discharge currents I1 and I2 flow out to the battery cells 1 and 2 and charge and discharge the battery cells 1 and 2.